Electric heater



March 14, 1939.

C. A. NICKLE El AL ELECTRIC HEATER Filed Nov. 18, 1956 Invenicirs: Ciifi ord A. Nickie, Gerald REiro by b flaw 6.49

y Their Attorney.

atented Mar. 14, 1939 UNITED STATES PATENT OFFICE ELECTRIC HEATER.

Application November 18, 1936, Serial No. 111,404

2 Claims.

This invention relates to electric heaters, more particularly to electric heaters of the sheathed type, and it has for its object the provision of an improved heater of this character.

5 This invention has particular application to electric heaters of the sheathed type, such as described and claimed in the U. S. patent to C. C. Abbott, No. 1,367,341, dated February 21, 1921. Briefly, this heater comprises a resistance heating element inclosed in a metallic sheath, and embedded in and supported in spaced relation with the sheath by a densely packed layer of heat refractory and heat conducting, electrically insulating material. It is to be understood, however, that this invention is not limited to the specific heater disclosed in this patent, but has more general application.

The heat refractory insulating material that has found wide application in sheathed heating units is powdered magnesium oxide, and heating units provided with this material have, in the main, been quite satisfactory. However, in certain cases they have a tendency to lose their electrical resistance with age; that is, the insulation resistance of the heater does not remain constant, but decreases with age.

This invention contemplates the provision of an improved electric heater, which has a very high initial electrical resistance and which maintains the resistance at a comparatively high value throughout the life of the heater.

In accordance with this invention, the resistance conductor of the heater is embedded in and supported in spaced relation with the walls of its sheath by a mass of densely packed insulating material formed of a mixture of magnesium oxide and boron nitride. The mass should consist mainly of magnesium oxide, but have mixed therewith a minor quantity of the boron nitride. The proportion of the boron nitride may vary widely from relatively low percentages, such as 1% by volume, to much higher percentages, such as 25% or more by volume.

For a more complete understanding of this invention, reference should be had to the accompanying drawing in which the single figure is a fragmentary view in elevation of an electric heater embodying this invention, portions of the heater being shown in section so as to illustrate certain structural details.

Referring to the drawing, this invention has been shown in one form as applied to an electric heater of the type described and claimed in the above-mentioned Abbott Patent No. 1,257,341.

I As shown, the heater comprises a helical resistance conductor Ill mounted within and substantially centrally of a tubular metallic sheath II. The resistance conductor and the sheath preferably will be formed of nickel-chromium alloys.

The resistance conductor [0 is embedded in and. supported in spaced relation with the sheath by a mass (2 of heat refractory and heat conducting, electrically insulating material. The mass of insulating material is compacted to a. hard dense mass in any suitable manner, as by reducing the diameter of the sheath ll after the resistance conductor and insulating material have been assembled with the sheath. The reducing operation may be accomplished by swaging or rolling.

In accordance with this invention, the mass of insulating material I2 is formed of a mixture of powered magnesium oxide and boron nitride.

It has been found that the proportion of boron nitride may vary widely from relatively small quantities, such as 1% or less by volume, to larger quantities, such as 25% or more by volume. An addition of boron nitride of 1% by volume gives a marked improvement in the insulating characteristic of the heater, but the greatest improvement is obtained by using the larger percentages.

It is not known definitely why the addition of minor quantities of boron nitride, such as 1% by volume, so materially improves the electrical characteristics of the heater by increasing its initial insulation value and by maintaining a fairly high insulation value through the life of the heater. At the present time, it is thought that these characteristics are obtained by the boron nitride covering or coating the particles of the magnesium oxide, and the surfaces of resistance conductor l0 and the sheath II, but this has not been definitely established. It has been definitely established, however, that the resistance characteristics of the heater are markedly improved by the addition of even this very small quantity of boron nitride.

The insulation characteristics improve by increasing the proportion of boron nitride. We have discovered, however, that very large proportions of boron nitride. weaken the insulation mechanically; that is, in cases where large percentages of boron nitride are used it is possible that the insulating layer l2 will not have a suflicihtly high mechanical strength to support the resistance conductor 10 centrally of the sheath, with the result that portions of the resistance conductor may work away from the central position toward the wall of the sheath. This is particularly apt to happen when the sheath is bent from its straight shape into various curved .iorins. Furthermore, it has been discovered that mixtures containing very high percentages of boron nitride do not work well mechanically, that is, do not lend themselves very satisfactorily to the swaging operations used to reduce and elongate the sheath to compact the insulating mass. It has been found that in such cases the insulating layer will flow as the sheath is reduced and will not compact to the hard dense mass desired. The proportion of boron nitride should be so small, or conversely, the percentage of magnesium oxide should be sufliciently large to give mechanical strength to the insulating mass I2, and to render it susceptible to swaging operations. At the present time, it is believed that the best results will be obtained if the percentages of boron nitride be not much greater than 25% by volume.

It will be understood that the resistance conductor ll) of the finished heater will have its ends secured to terminals l3 which, as shown, have their inner ends embedded in the insulating mass Ii so as to be supported in spaced relation to the sheath.

While we have shown a particular embodiment of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we, therei'ore, contemplate by the appended claim-is it cover any such modifications fall within. tut true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric heater comprising a casing, a resistance conductor in said casing, and a mass of heat-refractory and heat-conducting electrically-insulating material embedding said resistance conductor and supporting it in spaced relation with said casing formed mainly oi! magnesium oxide and having a minor quantity of boron nitride, the proportion of boron nitride in said mass being-at least 1% by volume and not more than 25% by volume.

2. A sheathed electric heater capable of being bent into various shapes comprising a metallic sheath, a helical resistance conductor within said sheath and a densely compacted mass of powdered magnesium oxide and boron nitride embedding said resistance conductor and holding it in spaced relation with said sheath, the proportion of boron nitride in said mass ranging from approximately 1% to approximately 25% by volume.

CLIFFORD A. NICKLE. GERALD R. BROPHY. 

